The present invention relates to a method of damping mechanical vibrations in a vehicle according to the characteristics of Claim 1.
It is known that vehicles having a hybrid drive have a drive train with an internal-combustion engine and at least one electric machine (electric driving motor). For a further reduction of the fuel consumption, internal-combustion engines with exhaust gas turbochargers are increasingly considered which, in comparison to conventional internal-combustion engines, operate at lower rotational speeds and have fewer cylinders (for example, 3-cylinder engines instead of the previous 4-cylinder engines, or 4-cylinder engines instead of the previous 6-cylinder engines) while the performance is essentially the same or higher. The lower the rotational speed level and the number of cylinders of an internal-combustion engine, the larger the rotational non-uniformity resulting from the combustion.
In order to avoid a transmission of such system-caused rotational non-uniformities into the drive train or onto the vehicle body, special measures have to be taken.
A first approach in this respect is described in German Patent Document DE 10 2005 001 047 A1, specifically in connection with a hybrid vehicle. The hybrid vehicle has an internal-combustion engine having several cylinders and an electric machine provided for generating vehicle propulsion. For saving fuel, individual cylinders can be “switched off” during the operation of the internal-combustion engine. They will then move along unfired. Naturally, a cylinder switch-off is connected with an increase of the rotational non-uniformity of the torque supplied by the internal-combustion engine. In German Patent Document DE 10 2005 001 047 A1, a “degree” is determined for the rotational non-uniformity and, as a function of this degree, the electric machine is activated by the electric motor or generator such that the increase of the rotational non-uniformity caused by the cylinder switch-off is reduced. Details for controlling or regulating the electric machine are not given in German Patent Document DE 10 2005 001 047 A1.
It is an object of the invention to indicate a method of damping mechanical vibrations in a (hybrid) vehicle, which can be implemented by means of comparatively low expenditures.
The starting point of the invention is the idea of subjecting a signal describing the current vibration behavior of at least one vehicle component to a signal analysis and generating, by the electric driving motor of the (hybrid) vehicle, for at least one “interfering frequency” contained in the signal, a compensation torque counteracting this interfering frequency.
The term “vehicle component” used in the following description and in the claims should be interpreted extremely broadly. It may relate to an individual component as well as to a component composite, formed by a plurality of components, for example, to a vehicle body, to a functional unit, as for example, the electric machine, or the like.
“The signal describing the current vibration behavior of the vehicle component” may, for example, be an acceleration signal, for example, an acceleration signal which describes the acceleration (vibration) of the vehicle structure (vehicle body) or of a component of the vehicle body.
As an alternative, a rotational speed of a vehicle component, such as the rotational speed of the internal-combustion engine or that of the electric machine or the like, could also be used as the signal describing the current vibration behavior of a vehicle component. Furthermore or as an alternative, the current vibration behavior could also be described by a signal which describes a measured or approximately mathematically (for example, by a state observer) determined drive torque of a vehicle component (such as an axle torque).
According to the invention, it may be provided that the at least one interference frequency, which is examined within the scope of the signal analysis (for example, by a discrete Fourier transform), is a predefined interference frequency that is specific of the type of internal-combustion engine used. The at least one interference frequency may particularly be a so-called “engine order”. By means of the signal analysis, frequencies beyond the engine orders could also be determined and damped.
As known, as a function of the construction of an internal-combustion engine (in-line engine, V-engine) and the number of its cylinders, there are, in addition to the basic frequency of the internal-combustion engine, certain so-called “harmonics”. The frequencies of the harmonics (interference frequencies) frequently are multiples of the basic frequency of the internal-combustion engine. In the case of some internal-combustion engines, there is also a harmonic whose frequency corresponds to half the basic frequency. These interference frequencies have the name of so-called (“engine) orders”. The ordinal number indicates the factor with respect to the basic frequency. Thus, the 0.5th engine order corresponds to that interference frequency curve whose frequency corresponds to half the rotational engine speed. The third engine order corresponds to the interference frequency curve with three times the rotational engine speed.
According to the invention, it may be provided that—depending on the type of the just present internal-combustion engine—one compensation torque respectively counteracting the assigned interference frequency is calculated for one, two, three or more engine orders and is superposed on the drive train by way of the electric driving motor.
As an alternative or in addition, it may also be provided that at least one interference frequency or several interference frequencies are determined by a Fourier analysis. The exact method of carrying out the Fourier analysis is sufficiently known to the person skilled in the art from the prior art and therefore does not have to be explained in detail. Corresponding information is contained, for example, in Brigham, E. Oran (1988). The Fast Fourier Transform and Its Applications. Englewood Cliffs, N.J.; Prentiss Hall, ISBN 0-13-307505-2.
If the signal describing the current vibration behavior of the at least one vehicle component is an analog signal, it may be provided that this signal is sampled with a predefined sampling frequency and the resulting sampling signal is subjected to a discrete Fourier transform (DFT). By examining the frequency response of this signal, those frequencies can easily be determined which cause particularly disturbing rotational non-uniformities or vibrations or noises in the vehicle.
It can frequently be observed that the signal describing the current vibration behavior of the at least one vehicle component does not contain only a single particularly dominant interference frequency but several interference frequencies. Correspondingly, it may be provided that, by an electronic system, compensation torques assigned to several interference frequencies contained in the signal, are calculated, and these compensation torques are superposed on the compensation torque to be generated by the electric driving motor of the hybrid drive.
The signal form of the compensation torque to be generated for a defined interference frequency can be generated as follows: By a frequency generator, two periodic “test signals” phase-shifted by 90° are generated which each have the same frequency as the considered interference frequency to be compensated.
By a predefined transmission function, which describes the transmission behavior between the electric driving motor of the (hybrid) vehicle and the at least one vehicle component whose vibration is monitored, a “first test output signal” is generated from the first test signal, and a “second test output signal” is generated from the second test signal.
Subsequently, by a “recursive least mean square algorithm” sufficiently known to the person skilled in the art, a so-called adaptation coefficient is determined. A recursive least mean square algorithm is known, for example, from Haykin, S. Adaptive Filter Theory. 3rd ed. Englewood Cliffs, N.J.: Prentice Hall, 1996.
The first or the second test output signal and the signal describing the current vibration behavior of the at least one vehicle component form input signals for the least mean square algorithm.
A “first adaptation coefficient” is generated from the first test output signal and the signal describing the current vibration behavior of the at least one vehicle component. From the second test output signal and the signal describing the current vibration behavior of the at least one vehicle component, a “second adaptation coefficient” is generated.
The first adaptation coefficient is multiplied by the assigned first test signal, and the second adaptation coefficient is multiplied by the assigned second test signal to a first and second correction term respectively. The sum of the two correction terms describes the signal form of a compensation torque damping or absorbing the considered interference frequency.
In the above-described manner, compensation torque courses assigned to several interference frequencies can be calculated by an electronic system. These can then be superposed on the compensation torque to be generated by the electric driving motor of the hybrid drive, i.e., added to the above-mentioned (total) compensation torque, which is superposed from the drive of the vehicle by the electric driving motor. The electric driving motor is therefore activated such that it generates a (total) compensation torque, which damps or ideally even completely absorbs the individual interference frequencies.
Summarizing, particularly the following advantages can be achieved by the invention:                The described method provides a simple possibility for the damping of vibrations in the drive train for hybrid vehicles;        the method can be easily applied because only a few parameters have to be adjusted;        the method requires only a few sensor signals. A high-expenditure communication between control devices is therefore eliminated;        the algorithm has a low susceptibility to signal traveling times.        
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.